Looks like there is a permissions issue or potentially a file system problem. The best way to solve this problem is to insert the system disc that came with your iBook G4 (or any 10.5 retail Leopard disc), and boot off of it by starting up while holding down the letter C. Once you've booted to the disc, continue through the normal pre-installation process until you are able to use the Utilities menu, which allows you to access Disk Utility. From Disk Utility, you can click on your hard drive (default is named "Macintosh HD") in the list on the left, and then click "Repair Disk Permissions." This may take a while (up to 45 minutes in some cases). It may also be worth clicking "Repair Disk" as well, to make sure that there aren't any file system issues remaining.
After that, restart normally and see if you can log in.
Please let me know how it goes. Good luck!